What it means
The first 100-degree day is the Valley’s unofficial changing of the seasons. Spring here does not end with a date; it ends the afternoon the thermometer at Sky Harbor, the official station for the whole metro, touches 100 for the first time since October. Television stations run countdown graphics, offices run betting pools, and locals say the number to each other the way people elsewhere mention the first snow: “Hit a hundred today.” On average it happens around May 2. From there the city collects roughly 111 triple-digit days a year, and the number stops being news.
The number that stays news is 110. Locals do not fear 100; they adjust to it. The 110s, which usually begin in mid-June, are the stretch when the pool is bathwater, the steering wheel needs a towel, and the city’s rhythm flips to early morning and after dark. The summer resort playbook explains how the hotels price around it; the sunrise hiking rules explain how not to need a helicopter during it. This page is about the milestone itself and how to read it.
Where you’ll meet it
Local news from mid-April on, the best-time-to-visit guide, the packing list, and every conversation with a neighbor between the first 100 and the first storm.
Where the word came from
Nobody coined it. The phrase is the natural product of a city with one official thermometer and a long record. The National Weather Service’s Phoenix office keeps the books at Sky Harbor, and it is from those books that the averages come: a first 100 around May 2, a first 110 around June 11, and about 111 days a year at 100 or more. The milestone became a media ritual as the Valley grew, and the Weather Service’s own social media now announces it within minutes.
The record keeps moving in one direction. The earliest 100-degree reading stood for nearly four decades at March 26, 1988. On March 18, 2026, Phoenix beat it by eight days. The earliest 110 remains May 8, 1989. At the other end, a first 100 in late May now counts as a cool spring.
The 110s have their own ledger, and 2023 rewrote it. From June 30 through July 30, 2023, Phoenix recorded 31 consecutive days at 110 or hotter, obliterating the previous record of 18 straight days set in 1974; the streak ended on July 31 when the high topped out at 108. July 2023 averaged 102.7 degrees, including overnight lows, which Arizona’s state climatologist called the hottest month ever recorded in an American city. July 19 brought a low of 97, the warmest night on the books. By September 9, 2023 the city had logged 54 days at 110 or above, breaking the annual record of 53 from 2020, and the season added more before it was done. Then 2024 broke a different record: 100 degrees or hotter on 100 straight days by September 3, a run that started May 27 and left the 76-day streak of 1993 far behind.
What it is not
The first 100 is not an excessive heat warning, which the Weather Service issues for specific dangerous stretches and which triggers Phoenix’s trail closures. It is also not the same as dry heat: a 100-degree day in early May with a dew point of 20 is a different animal from a 100-degree evening in August with the monsoon moisture in. Locals who say they prefer 108 in June to 100 in August are not being contrary.
The fine print
Two caveats about the numbers. First, every one of them is Sky Harbor’s. The airport sits at about 1,100 feet in the middle of the urban heat island, surrounded by runways and freeways, and it runs warmer, especially at night, than most of Scottsdale. North Scottsdale’s foothills are a few hundred feet higher with far more open desert, so a 112 at the airport is often 109 or 110 at Pinnacle Peak. The city has no official station with a comparable century of records, so the airport’s number is the number, and Scottsdale residents simply apply a mental discount they rarely get to enjoy.
Second, “100-degree days” and “consecutive 100-degree days” are different records that get confused in headlines. The roughly 111 figure is an annual total; the 100 straight days of 2024 is a streak. Likewise, 2023’s 31-day run of 110s is a streak, while its 54-plus days of 110 is a seasonal count. Phoenix’s parks department, which closes trails on the hottest days, counted 121 days at 105 or above in 2024 and 69 days above 110, numbers that would have been unthinkable in the 1970s and that the department now plans around.
What locals do about it is the point of marking the day at all. The first 100 is the signal to service the air conditioner, re-time the irrigation, start hiking at dawn and stop scheduling anything outdoors after ten. The first 110 is the signal that the real season, the one the city is secretly proud of surviving, has begun.